One of the issues in the development and usage of broad beam ion sources is in the production of very uniform ion beam density profiles. Since electric discharge plasmas do not themselves have a uniform distribution of ion density, ion sources which utilize ions generated in the plasma typically have non-uniform ion beam density profiles. Though the discussion here references beam grids, for example for ion sources, it applies generally to any charged particle broad beam source including both positive and negative ion beam sources and electron sources.
In order to solve this problem and achieve higher uniformities of ion beam current densities, ion extraction grids of gridded ion sources have been developed with variations of grid open area fraction (grid transparency) over the entire grid pattern. Indeed many have discrete sections of grid patterns in each of which may be different hole-to-hole distances and/or different hole diameters. Such ion extraction grids have been provided as solutions for various applications. At higher requirements of ion beam current density uniformities, however, the boundaries of such discrete sections may still cause unwanted disturbances in ion beam current density uniformities.
FIG. 1 shows one example of an arbitrary conventional grid pattern which has multiple discrete radially defined zones for grid transparency (the concentric circles being indicative of, and demarcating the boundaries between zones). Within each zone, the grid design is typically filled with repeating patterns to obtain a constant grid transparency within that zone. At the boundaries of zones, however, the transition between one zone and another may not be smooth, resulting in local discontinuities in the grid hole density, and, if left uncorrected, in the beam current density. FIG. 2 shows typical irregularities at both radial and azimuthal zone boundaries (note, the small black circles show locations of holes if they were equally spaced). These zone boundaries may be radial and various azimuthal boundaries may appear as shown here or, depending on the particular design, there may be other boundaries where either the hole size or spacing changes discontinuously. (Note, the six areas unpopulated with holes that are observed at the second radial boundary from the center in FIG. 1 are a result of other design features not relevant to the subject of this disclosure.) Conventionally, any adjustment to smooth the transition at the boundaries has been done by design personnel on a hole-by-hole basis. Shown in FIG. 3 is a distribution of grid transparency using an arbitrary unit as a function of radius in another arbitrary conventional grid design. The scattered data points in FIG. 3 are associated with boundaries where the patterns do not match and holes have been manually adjusted.